In general, in order to enhance fuel consumption efficiency of an engine, for example, there is known a technique of, when a vehicle is stopped at an intersection or the like, automatically stopping the engine under a predetermined automatic stop condition, and then automatically restarting the engine under a predetermined restart condition. Here, when the engine is automatically stopped, an operation of a mechanical oil pump that supplies operating oil to a hydraulic control device in an automatic transmission using power of the engine is also stopped, and hydraulic pressure in the hydraulic control device is decreased by an amount of the operating oil leaking. In this state, a starting frictional engagement device (a clutch or a brake for starting) which is provided in the automatic transmission and is engaged at the time of starting the vehicle is not supplied with the hydraulic pressure needed for the engagement from the hydraulic control device and is in a released state. Thereafter, when the vehicle is started by automatically restarting the engine, if the engagement of the starting frictional engagement device is not promptly performed, a rise in the number of the engine rotations occurs, or the starting frictional engagement device is engaged after the number of engine rotations increases, which causes vibration and shock. Therefore, responsiveness of the vehicle at the time of starting is degraded.
In JP-A-2000-46166 (Patent Document 1), when an engine is automatically stopped, an electric oil pump which is externally attached is driven by an electric motor to supply operating oil discharged from the electric oil pump to a hydraulic control device in an automatic transmission. When the vehicle is started by automatically restarting the engine, hydraulic pressure needed for engaging the starting frictional engagement device in the automatic transmission is ensured by the operating oil supplied from the electric oil pump to the hydraulic control device.
In addition, in JP-A-8-14076 (Patent Document 2), when an engine is driven, operating oil discharged from a mechanical oil pump driven by power of the engine is accumulated by an accumulator in advance, and when the engine is automatically stopped, the operating oil accumulated in the accumulator is supplied to a hydraulic control device in an automatic transmission. When a vehicle is started by automatically restarting the engine, hydraulic pressure needed for engaging a starting frictional engagement device in an automatic transmission is ensured by the operating oil supplied from the accumulator to the hydraulic control device.
In addition, in JP-A-11-159366 (Patent Document 3), when an engine is automatically stopped, operating oil accumulated by an accumulator is supplied to a hydraulic control device in an automatic transmission. Here, in Patent Document 3, the accumulation of the operating oil in the accumulator is performed by driving an electric oil pump using an electric motor and supplying the operating oil discharged from the electric oil pump to the accumulator. When a vehicle is started by automatically restarting the engine, hydraulic pressure needed for engaging a starting frictional engagement device in an automatic transmission is ensured by the operating oil supplied from the accumulator to the hydraulic control device.
When the engine is automatically stopped, the hydraulic pressure in the hydraulic control device in the automatic transmission is decreased by an amount of the operating oil leaking. In order to compensate for the decrease in the hydraulic pressure in the hydraulic control device, it is preferable that the operating oil be continuously supplied to the hydraulic control device. In addition, when the vehicle is started by automatically restarting the engine, in order to promptly engage the starting frictional engagement device in the automatic transmission, it is preferable that the pressure of the operating oil supplied to the starting frictional engagement device be promptly increased.
The oil pump is suitable for continuously supplying the operating oil for a long time. However, in order to supply high-pressure operating oil instantly, there is need to increase a discharge capacity, which results in an increase in size. Therefore, the oil pump is not suitable for supplying the high-pressure operating oil instantly. As disclosed in Patent Document 1, when the vehicle is started by automatically restarting the engine, in order to promptly perform the engagement of the starting frictional engagement device using only the operating oil discharged from the electric oil pump, there is need to increase a discharge capacity of the electric oil pump. As a result, the electric oil pump size is increased, and power consumption is also increased.
In addition, the accumulator is suitable for supplying high-pressure operating oil promptly; however, an amount of the operating oil accumulated is limited, so that the accumulator is not suitable for continuously supplying the operating oil for a long time. As disclosed in Patent Documents 2 and 3, when the engine is automatically stopped, in a configuration in which only the operating oil accumulated in the accumulator is supplied to the hydraulic control device in the automatic transmission, if a stop time (a time from the automatic stop to the automatic restart) during the automatic stop of the engine is prolonged, most of the operating oil accumulated in the accumulator is consumed to compensate for the leaking operating oil (reduction in hydraulic pressure) in the hydraulic control device. As a result, when the vehicle is started by automatically restarting the engine, only with the operating oil accumulated in the accumulator, an amount of the operating oil needed for engaging the starting frictional engagement device is not met. On the other hand, when a large amount of the operating oil is to be accumulated in the accumulator in order to allow for an extension of the stop time of the engine, a large and heavy-weight accumulator is needed, thereby degrading the mounting performance of the device. In Patent Document 3, the accumulation of the operating oil in the accumulator is performed by supplying the operating oil discharged from the electric oil pump to the accumulator. However, in order to accumulate the high-pressure operating oil in the accumulator, the size of the electric oil pump needs to be increased, and power consumption is also increased.
A need thus exist for a hydraulic supply device which is not susceptible to the drawback mentioned above.